1. Field of the Invention
The invention relates to a separator for use in a fuel cell.
2. Description of the Related Art
A solid polymer electrolyte type fuel cell battery is formed by a stack of membrane-electrode assemblies (MEAs) and separators. A membrane-electrode assembly includes an electrolyte membrane formed by an ion exchange membrane, an electrode (anode, or fuel electrode) formed by a catalytic layer that is disposed on a surface of the electrolyte membrane, and an electrode (cathode, or air electrode) formed by a catalytic layer that is disposed on another surface of the electrolyte membrane. An anode-side diffusion layer and a cathode-side diffusion layer are provided between the membrane-electrode assembly and separators. A separator has a fuel gas channel for supplying a fuel gas (hydrogen) to the anode, and an oxidizing gas channel for supplying an oxidizing gas (oxygen, or air in ordinary cases) to the cathode. Each separator further has a coolant channel for passing a coolant (that is normally cooling water). A cell is formed by stacking a membrane-electrode assembly and separators, and a module is formed by at least one cell. Modules are stacked to form a cell stack. Terminals, insulators and end plates are disposed on two opposite ends of the cell stack in the stacking direction. The cell stack is clamped in the cell stacking direction, and is fixed through the use of fastener members (e.g., tension plates) that extend outside the cell stack in the cell stacking direction, and bolts and nuts. In this manner, a stack is formed. On the anode side of each cell, a reaction occurs in which hydrogen is separated into hydrogen ions (protons) and electrons. The hydrogen ions migrate through the electrolyte membrane to the cathode side. On the cathode side, a reaction mentioned below occurs in which water is produced from oxygen, hydrogen ions and electrons (i.e., the electrons produced on the anode of the adjacent MEA come to the cathode through the separator, or the electrons produced on the anode of the cell disposed at an end in the cell stacking direction come to the cathode of the cell at the opposite end via an external circuit).
Anode side: H2→2H++2e−
Cathode side: 2H++2e−+(½)O2→H2O
For example, Japanese Patent Application Laid-open No. 2000-82482 discloses a fuel cell as schematically illustrated in FIG. 4 in which a gas inlet (e.g., an oxidizing gas inlet 28a) and a gas outlet (e.g., an oxidizing gas outlet 28b) are located at the same side of a separator 18 (the same edge side of a rectangular separator), and a gas channel (e.g., an oxidizing gas channel 28) has a serpentine channel configuration having a plurality of turnaround portions (e.g., a turnaround portion 28c of the oxidizing gas channel).
However, related-art fuel cells having serpentine gas channels have the following drawbacks. (1) Due to the great lengths of gas channels, the gas concentration greatly differs between the gas inlet and the gas outlet; that is, on the downstream side, the gas concentration decreases, and the state of electric power generation degrades. In Japanese Patent Application Laid-open No. 2000-82482, this problem is solved by providing an intermediate manifold. However, the provision of the intermediate manifold impedes size reduction of the fuel cell. (2) The IV characteristic (current-voltage characteristic) of the fuel cell deteriorates sharply and significantly in a high current density region. Causes for this deterioration are considered to be gas supply shortage and flooding caused by large production of water.